Electric cables and wires are generally composed of one or several polymer layers extruded around an electric conductor(s). The electric conductor is usually coated first with an inner semiconducting layer followed by an insulating layer, and then an outer semiconducting layer. To these layers further layers may be added, such as a water barrier layer and a surrounding sheath layer (jacketing layer) applied on the outside of the said layers.
The insulating layer and the semiconducting layers normally consist of a polymer composition comprising a crosslinked polyolefin. Crosslinking substantially contributes to improve such properties of the polymer as its heat and creep resistance, mechanical strength, chemical resistance, and abrasion resistance.
Common polymeric materials for wire and cable applications comprises ethylene homo- and/or copolymers and propylene homo- and/or copolymers, including ethylene-propylene-elastomers. Normally, the insulating layer and the semiconducting layer comprises crosslinked ethylene homo- and/or copolymers, herein referred to as ethylene (co)polymers. LDPE (low density polyethylene), i.e. polyethylene prepared by radical polymerisation at high pressure, crosslinked by adding peroxide, for instance dicumyl peroxide, is today the predominant cable insulating material for power cables.
Cross-linked polyolefins, such as crosslinked ethylene homo- and/or copolymers and propylene homo- and/or copolymers, are also extensively used for pipes, such as water pipes, gas pipes, sewage pipes, coated steel pipes and aluminum multilayer pipes.
Crosslinking can be brought about by adding free-radical-forming agents (also called crosslinking or curing agents), such as peroxides, to the polymer composition prior to or during extrusion of the cable or pipe and the crosslinking is initiated by heating in a subsequent vulcanization step, such that the peroxide is decomposed to form free radicals. These free radicals introduce the crosslinks in the material and thus build up the network structure.
In general, the degree of unsaturation of polyolefins is dependent on specific conditions chosen for the polymerisation process. This is true for high pressure as well as low pressure conditions. Normally, polyethylene produced by radical polymerisation, so-called low-density polyethylene, LDPE, has a low degree of unsaturation in the order of 0.1 vinyl groups (generally up to about 0.37 double bonds) per 1000 carbon atoms. However, in many situations, it is desirable to use polymers having a higher degree of unsaturation, which may serve as seat for chemical reactions, such as the introduction of functional groups into the polymer molecule or the cross-linking of the polymer.
In WO 9308222 it is described how the unsaturation of an ethylene copolymer can be increased by high pressure radical polymerisation of ethylene and a specific type of polyunsaturated monomer, such as an α,ω-alkadiene. One double bond of this polyunsaturated compound is polymerised into the polymer chain while the other double bond or bonds do not react and instead increase the unsaturation of the polymer. The non-reacted double bond(s) will be positioned at the end of short branches, thus providing so-called pendant vinyl groups, at the site in the polymer chain where the polyunsaturated compound was incorporated by polymerisation, such that the unsaturation is uniformly distributed along the polymer chain in essentially random copolymerisation. The increased amount of unsaturation of the LDPE copolymer increases the crosslinking response when combined with a crosslinking agent.
In WO 9635732 it is described how the unsaturation of an ethylene copolymer can be increased by high pressure radical polymerisation of ethylene and a certain type of polyunsaturated α,ω-divinylsiloxanes. The prepared ethylene copolymers have enhanced water tree resistance and an increased degree of unsaturation.
In WO 9745465 it is described how the unsaturation of an ethylene copolymer can be increased by high pressure radical polymerisation of ethylene and a certain type of polyunsaturated α,ω-divinylether.
As apparent for persons skilled in the art, the above mentioned ethylene copolymers can be cross-linked by subjecting the copolymer to an elevated temperature in the presence of a cross-linking agent.
Generally, these types of ethylene copolymers start to crosslink at about 160° C. depending on the type of crosslinking agent used. The temperature of the vulcanization tube is usually up to about 400° C.
A crosslinking agent is usually defined to be any compound capable to generate radicals which can initiate a crosslinking reaction. Normally, the crosslinking agent contains at least one —O—O— bond or at least one —N═N— bond. Peroxides, such as dicumylperoxide, are often used as crosslinking agents.
During the crosslinking step, by-products are generated due to decomposition of the crosslinking agent.
For instance, dicumylperoxide typically generates methane, acetophenone and cumyl alcohol upon decomposition.
Most by-products are captured within the cable and the volatile fraction thereof has to be removed in a subsequent so-called degassing step. The more by-products generated, the longer the degassing time and/or the higher the degassing temperature. However, mild degassing conditions would be preferred. Milder degassing conditions would also reduce the risk of damaging the cable core layers during the degassing step.
As used herein, volatile by-products comprise any low-molecular compounds which are captured within the polymer composition after the crosslinking step and are removable by thermal treatment at a temperature low enough to avoid significant degradation of the polymeric material.
Moreover, the by-products captured within the cable are usually of polar nature which means that they will have a negative impact on the electrical properties of the cable.
Thus, as evident from above, it would be a great advantage if the generation of these by-products could be reduced or avoided.
U.S. Pat. No. 4,027,080 describes a method for crosslinking of a polyolefin, such as an unmodified high density polyethylene, in the presence of a crosslinking agent without peroxide groups and which give free radicals by splitting along the carbon-carbon bonds, such as 3,4-dimethyl-3,4-diphenylhexane.
U.S. Pat. No. 6,967,229, U.S. Pat. No. 5,118,763, and EP 0430092 describe the use of so-called carbon-carbon initiators for crosslinking of polybutadiene and/or other elastomeric polymers, such as EPDM.
Mention can also be made of JP 06-275129 which discloses a composition comprising a thermoplastic resin, such as polyethylene, 3,4-dimethyl-3,4-diphenylhexane and a multi-functional compound, such as triallylcyanurate.